This application is a 371 of International Application PCT/EP97/06065, filed on Nov. 3, 1997.
The invention relates to the use of polyaspartic acids and their mixtures with surfactants and emulsifiers for inhibiting or delaying deposit formation by sparingly soluble organic and inorganic components in the feed of membrane processes.
Membrane technology plays an important role in the separation of fluidized systems. Obtaining drinking water from sea water by means of reverse osmosis as well as the breaking up of products by means of ultrafiltration and nanofiltration are now among the established processes in industry.
In membrane processes, as a rule dilute solutions are concentrated and organic solvents, water or salt solutions are separated off. Either valuable substances or pollutants in concentrated and possibly low-salt solutions are obtained, with the result that subsequent storage, transport, disposal and further processing can be carried out more economically. In the case of wastewater treatment, the aim of membrane treatment is to obtain the major part of the volume as permeate in an uncontaminated or only slightly contaminated form, for example for reuse. The concentrated retentate can be worked up at relatively low cost for valuable substances still present or can be disposed of more economically in this concentrated form, for example by incineration.
The area of membrane processes includes very different processes. The membranes and their technical designs, the modules, also differ correspondingly. Commercial membranes are produced, for example, from organic materials, such as polysulphone, cellulose acetate, polyamide or PVDF, or inorganic materials, such as TiO.sub.2, ZrO.sub.2 or Al.sub.2 O.sub.3 ; they are used in the form of capillary, tubular or flat membranes.
Industrially relevant membrane separation processes are predominantly operated as crossflow filtrations. High wall shear stresses, realized by high flow velocities and special module constructions, are intended to minimize or inhibit soiling of the membrane. In general, however, a decrease in the permeate output due to fouling or the accumulation of material on the membrane, is nevertheless found during the concentration of feedstreams in industrial membrane processes.
Scaling, the encrustation of the membrane by inorganic salts as a result of exceeding their solubility limit, is a special case of fouling. The calcium and magnesium carbonates, hydroxides, phosphates, sulphates and fluorides due to the hardness of the water may be primarily mentioned here as inorganic salts. In wastewater treatment, heavy metal hydroxides, such as, for example, iron and chromium hydroxides, constitute an additional problem. Scaling is always to be expected if high permeate yields are strived for in a process, such as, for example, in wastewater concentration and the recovery of pure water and drinking water. However, this phenomenon can of course also occur in desalination and concentration of product solutions. Membrane processes considered here are ultra- and nanofiltration, reverse osmosis, dialysis and perevaporation.
Fouling and scaling as a special instance of fouling result in the permeate output of a membrane plant finally decreasing to an uneconomically low level. From time to time, the feedstream must therefore be shut off and the membrane cleaned. However, such a cleaning process has several disadvantages. Cleaning means interrupting operation. Continuous operation can thus be maintained only by keeping a parallel membrane apparatus in reserve. Furthermore, depending on the type of deposits, it is necessary to use chemical cleaning agents which frequently contain poorly biodegradable surfactants and complexing agents and have to be disposed of separately. Finally, the entire deposit is in general not removed during cleaning and the membranes therefore seldom achieve their original permeate flow when they are used again.
If in a process scaling is expected, pretreatment measures, for example, by the use of ion exchangers which are known from the field of water softening, can be taken to inhibit said scaling. Furthermore, the controlled introduction of solids particles in the seeding and fluidized-bed technique permits physical control of membrane encrustation in the case of certain scalant and module systems (Chem.-Ing.-Tech. 59 (1987) 187). Hydroxide deposits can often be avoided by establishing suitable pH values. Furthermore, there has been no lack of attempts to add complexing agents, such as NTA and EDTA, to the feedstream to be processed. In addition to the not uncontested disadvantageous ecotoxicological properties, the complexing agents must be added in equimolar amounts. In contrast to the complexing agents, dispersants, so-called threshold inhibitors, can be effectively used in substoichiometric amounts for inhibiting or delaying fouling and scaling in membrane processes.
In Desalination 54, 263-76 (1985), cited in Chem. Abstracts, 104, 56 102, polyphosphates, phosphonates, polystyrenesulphonates, polyacrylamides and polyacrylates are investigated with regard to the scale-inhibiting action. In U.S. Pat. No. 5,256,303 the inhibition of calcium sulphate crystallization and of deposition in feedstreams which are passed through a membrane system is investigated. N-substituted polyacrylamides and phosphonobutane-1,2,4-tricarboxylic acid are used as scale inhibitors. A method for inhibiting the crystallization of sulphates in aqueous systems is described in EP 0 705 794. One or more polyamino acids and one or more inorganic phosphates are used but have to be disposed of by a complicated procedure because of eutrophication in waters from the main outfall. The preparation of high molecular weight polyaspartic copolymers, which can likewise be used as scale inhibitors in various industrial and hygiene areas, is described in U.S. Pat. No. 5,286,810; reverse osmosis membranes, too, are mentioned, without specific details. Mixtures of polyaspartic acids and their derivatives with other polycarboxylic acids and their use in water treatment are described in U.S. Pat. No. 5,525,257. Polyacrylates, polymaleates and polysulphonates are mentioned as polycarboxylic acids; here too, reverse osmosis is mentioned without further information. According to U.S. Pat. No. 5,466,760, copolymers of polysuccinimide from maleic acid, ammonia and a polyamine are used as inhibitors of salt deposits. EP-B 530 358 (=U.S. Pat. No. 5,373,086) describes a special polyaspartic acid composition which is obtained by heating pulverulent L-aspartic acid to at least 188.degree. C. and condensation, further heating to at least 216.degree. C. until at least 80% of the polysuccinimide formation has taken place and subsequent hydrolysis of the polysuccinimide, is present to an extent of over 50% in the .beta.-form and has a molecular weight of 1000 to 5000 (weight average), which is used for inhibiting precipitation of CaCO.sub.3 or Ca.sub.3 (PO.sub.4).sub.2. This special polyaspartic acid is said to be capable of being used in a wide industrial area from process water treatment to oil production; reverse osmosis is also mentioned in passing, without further information.
In summary, it can therefore be said that, according to the prior art to date, phosphonates and polyacrylates were used for scale inhibition in membrane processes. Polyaspartic acids are mentioned in numerous publications as useful scale inhibitors. However, there were to date no results which demonstrate the usability of polyaspartic acids in membrane processes. It is known to a person skilled in the art that effective scale inhibition always depends on the total system, i.e. all components and conditions involved. Particularly in membrane processes, many interfering chemical and physical factors are present which make it unlikely that a scale inhibitor investigated with a good result in another field of use will also be effective precisely in membrane processes.